1. Field of the Invention
The invention generally relates to a compact electronic display system. More specifically, the invention relates to a compact electronic display system which provides a virtual image of a microdisplay that has been magnified using two stages of magnification optics where one of the magnification optics is reflective and is used to fold the optical train of the electronic display onto itself.
2. Description of Related Art
A continuing objective in the field of electronics is the miniaturization of electronic devices. Most electronic devices include an electronic display. As a result, the miniaturization of electronic displays is critical to the production of a wide variety of compact electronic devices.
The purpose of an electronic display is to provide the eye with a visual image of certain information. This visual image may be provided as either a real image or a virtual image. A real image refers to an image which is observed directly by the unaided human eye. A photograph is an example of a real image. Electronic displays which provide a real image generally provide some form of display surface on which the real image is formed and viewed. A real image exists at a given location when, if a viewing surface is positioned at this location, a real image can be observed by the unaided eye. Examples of electronic displays which provide real images include liquid crystal displays, CRT monitors, and projection screens.
In real image electronic displays, the image viewed by the observer is an image created on or projected onto a display surface. As a result, the size of the display surface limits the size of the image that can be provided to the observer. Compact electronic devices, because of their small size, have limited space for a display surface and therefore are only able to accommodate a relatively small display image.
A relatively small display image creates a series of problems for the person viewing the image. The human eye is only able to resolve a limited amount of detail in an image. Two objects can be resolved by the eye as separate objects only when they are separated by a certain minimum angle as measured from the eye. The unaided human eye can resolve objects with an angle of separation of approximately 1-2 arc minutes or more.
The human eye also has difficulty focusing on source objects at very short distances from the eye without causing eye strain. The near point for the unaided human eye is defined as the closest distance that the eye can focus on an object without causing eye strain. The near point for the unaided human eye is about 25 cm for an average middle aged adult. The near point is much longer for an older adult and can be as long as 100 cm.
The display surface of a real image electronic display generally includes an array of small objects, called pixels, which form a real image on the display surface. For the reasons described above, the resolution of the unaided human eye and the near point of the eye determine the minimum pixel size that the unaided human eye can resolve. For an electronic display located at the near point for an average middle aged adult, the minimum separation between pixels that can be resolved by the unaided eye is about 75.times.10.sup.-4 cm, or 75 microns. In order to accommodate for variations in the human eye, the minimum separation between pixels in a real image electronic display should preferably be larger than the minimum resolvable pixel size.
For small real image electronic displays, the display must be viewed at a distance close to the near point of the eye in order to provide the observer with a significant amount of information. As a result, the observer must focus his or her eyes on the small display. The need to focus on a small display each time the electronic display is used creates discomfort and eventually, unwanted eye strain on the observer. The problem of eye strain becomes particularly acute when the observer is only intermittently focusing on the electronic display. It therefore is desirable to employ an electronic display which minimizes the observer's need to refocus his or her eyes in order to view the electronic display.
Another important property of the human eye which determines the utility of an electronic display is the eye's angular field of view of an image. The eye can see over a field of view of up to 100 degrees. The full field of view, as it is referred to herein, is the circular field of view around the axis of the eye having a diameter equal to the largest dimension of the image being viewed. However, beyond 10-15 degrees from the center of the field, the resolution degrades significantly. A comfortable field of view for normal electronic display surfaces is typically in the range of 20-40 degrees. For real image displays, the field of view is defined as the ratio between the largest dimension of the display surface and the distance from the eye to the display. An example of a display surface with such a field of view would be a TV screen with a 100 cm diagonal viewed at 150 cm. The human eye compensates for the lower resolution at the edges of the display surface by scanning the eye across the display. The scanning of the eye is called eye roll. The eye roll moves the pupil of the eye. The typical distance for the motion of the pupil of an adult is about 1 cm.
An optical system can produce both real and virtual images. Several examples of electronic displays that provide a real image were discussed above. By contrast to a real image, a virtual image is an image which, if a viewing surface were positioned at the location of the virtual image, no image would be observed by the eye. An example of a virtual image is the image of fine print viewed through a magnifying glass. The print not only appears larger, it also appears to be located substantially behind the surface where the print actually exists. By definition, a virtual image can exist at a location where no display surface exists. The size of the virtual image therefore is not limited by the size of a display surface. Virtual image electronic displays thus have the advantage of eliminating the need for a large display surface in order to produce a large electronic image.
A virtual image electronic display must initially form a source object which is then imaged by an optical system to create the virtual image. A substantial advantage of a virtual image electronic display is that the source object initially created may be as small as can be usefully reimaged by the optical system. As a result, virtual image electronic displays may effectively utilize very small microdisplays to form the source object. Pixel sizes may be as small as a few microns in diameter, a size which the unaided eye cannot resolve. Rather, in order to view the source object formed by the microdisplay, substantial magnification of the optical system is required.
A virtual image must be created by an optical system of some kind. In a real image electronic display, it is the eye and the viewing surface properties which determine the viewing parameters. By contrast, in a virtual image display, the optical system determines most of the viewing parameters.
There are three important parameters relating to the ease of viewing the image associated with virtual image displays. The first parameter is the far point which refers to the maximum distance from the eye which the optical system can be held and have the eye still see the entire virtual image. Optical devices which provide a far point which is a short distance from the optic are undesirable due to the inconvenience and discomfort associated with placing the eye in close proximity with the optic. It is therefore preferred that an optic provide a long far point in order to enable the magnified image to be viewed through the optic at a comfortable and convenient range of distances from the optic.
The second parameter relating to the ease of viewing a virtual image is the apparent angular width of the virtual image, commonly referred to as the field of view of the virtual image. The full field of view is defined as the ratio of the largest apparent dimension of the virtual image to the apparent distance to the virtual image. It is generally equivalent to the field of view for a real image display surface.
The third parameter relating to the ease of viewing a virtual image is the transverse distance that the eye may move with respect to the optical system and still have the eye see the entire virtual image through the optical system.
A variety of electronic display systems have been developed for providing a virtual image to the observer. Virtual image electronic display systems may generally be divided into two broad classes, on-axis display systems and off-axis display systems. An on-axis display system refers to a system having components symmetrical about a common optical axis. In a typical on-axis system, any of the component(s) forming the on-axis display system can be rotated about the optical axis without disturbing the display system.
On-axis display systems provide the advantage of producing virtual images with a minimal amount of aberrations. However, on-axis display systems have the disadvantage of being spatially inefficient due to the linear arrangement of the optical components.
By contrast, off-axis display systems refer to display systems where one or more components are positioned such that the symmetry around the optical axis is removed. Any optical system that includes tilted or displaced optics is an off-axis optical system as that term is used herein. By placing one or more components off-axis, off-axis display systems can be adapted to efficiently fit within the contours of the devices in which they are used. However, off-axis display systems have the disadvantage that redirecting an image off-axis introduces aberrations into the image which can significantly deteriorate the image quality produced. The image quality can frequently be enhanced using additional optical elements which reduce the significance of the aberrations. However, these additional optical elements add to the size, complexity and cost of the display.
Off-axis display systems commonly employ optical components having a reflective optical surface, such as a concave reflective mirror, in order to redirect the optical train off-axis. Examples of prior art off-axis electronic display systems employing a reflective element include U.S. Pat. No. 3,296,509, U.S. Pat. No. 4,717,248, U.S. Pat. No. 5,087,166, U.S. Pat. No. 5,157,503, U.S. Pat. No. 5,291,338, U.S. Pat. No. 5,305,124 and U.S. Pat. No. 5,357,372.
One problem associated with the use of a reflective optical surface is that the optical path on the object side of the surface and the optical path on the image side of the surface traverse the same physical space. This problem is generally avoided through the use of a second reflective surface, such as a beam splitter, or an optical element, such as an optical grating, which diverts the reflected image of the object off-axis.
The use of a beam splitter or optical grating to divert a reflected image off-axis has several disadvantages. Redirecting an optical image off-axis tends to introduce aberrations into the image which reduces the image quality of the image. Beam splitters and optical gratings also do not transmit all of the light to the observer and thus reduce the transmissivity and ultimately the brightness of the virtual image seen by the observer. For example, U.S. Pat. No. 4,859,031 teaches an optical collimating apparatus for use in a head-up display which transmits an image from an image source with a light efficiency of 4.95%. This low level of light efficiency is due, in part, to the use of a semi-reflective concave mirror, a circular polarizing filter and a transmitter/combiner which each cause 50% of the light intensity of the image to be lost.
A significant advantage associated with compact electronic displays is the fact that they are portable. It is therefore impractical and disadvantageous for a compact electronic display to rely on an external power source. The illumination source used in the electronic display generally requires the greatest amount of energy of the various components used in the electronic display. It is therefore important that the electronic display have an optical design which efficiently uses the illumination source used to form the virtual image.
A need also currently exists for an inexpensive, compact virtual image electronic display that is positionable within a small volume, that provides the observer with a large field of view, a virtual image with a significant degree of eye relief and a large translational distance. In particular, an electronic display system is currently needed which combines the image quality and light efficiency advantages of an on-axis display system with the spacial efficiency provided by off-axis display systems.